Exogamous marriage or exogamy is the custom of marrying outside a specified group of people to which a person belongs. In addition to blood relatives, marriage to members of a specific totem, clan(s) or other group(s) may be forbidden.

Different theories have been proposed to account for the origin of exogamy. Some are detailed here.

Edvard Westermarck said it arose in the aversion to marriage between blood relatives or near kin, that is, in horror of incest. From a genetic point of view, aversion to breeding with close relatives results in fewer congenital diseases, because, where one gene is faulty, there is a greater chance that the other - being from a different line - is of another functional type and can take over. Outbreeding thus favours the condition of heterozygosity, that is having two non-identical copies of a given gene.

J.F. McLennan [1] holds that exogamy was due originally to scarcity of women, which obliged men to seek wives from other groups, including marriage by capture, and this in time grew into a custom.

Morgan[2] and Howitt [How to reference and link to summary or text] maintain that exogamy was introduced to prevent marriage between blood relations, especially between brother and sister, which had been common in a previous state of promiscuity. Frazer says this is the true solution, that it really introduced group marriage, which is an advance to monogamy, and that the most complete record of this is the classificatory system of relationship. Lang, however, denies there is any group marriage, and says that so-called group marriage is only tribe-regulated licence. Hill-Tout [How to reference and link to summary or text] writes that exogamous rules arose for political reasons by marriage treaties between groups. Darwin denies primitive promiscuous intercourse, and says exogamy arose from the strongest male driving the other males out of the group. This is also the opinion of Lang, Atkinson, and Letourneau [How to reference and link to summary or text].

Claude Levi-Strauss introduced the "Alliance Theory" of exogamy, that is, that small groups must force their members to marry outside so as to build alliances with other groups. According to this theory, groups that engaged in exogamy would flourish, while those that didn't would die out, either literally or because they lacked ties for cultural and economic exchange, leaving them at a disadvantage. The exchange of men and/or women therefore served as a uniting force between groups.